What is the algorithm for querying data from two tables and what is its complexity?

Data query from two tables is usually performed using the JOIN operation in SQL. The algorithm depends on the type of join (INNER JOIN, LEFT JOIN, etc.) and the execution method (nested loops, hash join, merge join).

Example query:

SELECT a.*, b.*
FROM tableA a
JOIN tableB b ON a.key = b.key;

Algorithm:

1. For each record from the first table, find corresponding records in the second table based on the join condition.
2. The results are combined into a single row.

Complexity depends on the implementation:

• Nested loops join — O(N*M), where N and M are the sizes of the tables.
• Hash join — O(N + M), if data fits in memory and hashing is efficient.
• Merge join — O(N log N + M log M), if the tables are sorted.

Query optimization and algorithm choice depend on indexes, data volume, and database statistics.